A known and commercially-implemented EAS system is of a type involving a transmitting antenna and a receiving antenna placed about a controlled zone, such as the exit of a retail establishment. A transmitter furnishes signals to the transmitting antenna for transmission into the controlled zone and is energized from local power, in the United States at sixty Hertz and in Europe at fifty Hertz. While the transmitted signals are at a frequency substantially higher than the local power frequency, high harmonics of the local power frequency, often arising from other equipment in the vicinity of the controlled zone, e.g., cash registers, printers, neon lights, etc., can occur within the detecting frequency band of the receiver of the system. Such detecting frequency band, in the known system under discussion, encompasses the fundamental of the transmission frequency (the system operating frequency) and the second and third harmonics thereof.
EAS tags or markers affixed to articles are adapted, upon receipt of the transmitted signals, to return signals rich in the second harmonic and weak in the third harmonic of the system operating frequency. System alarm activation occurs when the receiver sees a rich second harmonic return in the absence of receipt concurrently of a fundamental frequency change or shift which is less than a predetermined level and of the third harmonic which is less than another predetermined level.
As will be appreciated, where the vicinity of the controlled zone has tag-extraneous presences of high levels of fundamental and/or third harmonic generators, i.e., generally interfering signals, the system may by its conditional logic come not to generate an alarm condition for a tag passing unauthorizedly through the controlled zone. Improved system insensitivity to tag-unrelated generation of such high level fundamental and third harmonic returns in the nature of interfering signals would manifestly improve the effectiveness of such known and other EAS systems.